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st: AW: RE: AW: RE: AW: Exclude some observations from a sample in a regression
From
"Martin Weiss" <[email protected]>
To
<[email protected]>
Subject
st: AW: RE: AW: RE: AW: Exclude some observations from a sample in a regression
Date
Wed, 7 Apr 2010 15:42:14 +0200
<>
" ...with -if 1 == 1- or -if 42 ==
42-, that is, it is a tautology, or always true."
That sounds like you did not specify Claude`s -if foreign==foreign-, but -if
_n==_n-, which is just as tautological, but can indeed lead to 42 being
compared to 42...
HTH
Martin
-----Ursprüngliche Nachricht-----
Von: [email protected]
[mailto:[email protected]] Im Auftrag von Nick Cox
Gesendet: Mittwoch, 7. April 2010 15:09
An: [email protected]
Betreff: st: RE: AW: RE: AW: Exclude some observations from a sample in a
regression
The overarching principle is that
... if <condition>
identifies the maximal subset that will be included. That doesn't stop
observations being excluded on other grounds, namely missing values of
-rep78- in these examples.
Note that
... if foreign == foreign
is no exclusion at all, being on all fours with -if 1 == 1- or -if 42 ==
42-, that is, it is a tautology, or always true. You may intend
... if foreign == "foreign":origin
which is quite different. See
SJ-4-4 dm0009 . . . . . . . Stata tip 14: Using value labels in
expressions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K.
Higbee
Q4/04 SJ 4(4):488--489 (no
commands)
tips for using value labels in expressions
Nick
[email protected]
Claude Francoeur
Thanks for showing me these different methods. I should have mentioned
that I do have missing values in my sample. I'm confused though
concerning the right command to use.
regress price weight length if foreign returns a regression using 22
observations since there are no missing cases
regress price weight length rep78 if foreign returns a regression using
21 observations since there is one missing value under rep78. This seems
to be contrary to Nick's comment, but I probably am missing something.
regress price weight length rep78 if foreign==foreign returns a
regression using all 69 non missing observations including domestic
cases. The if command does not seem to be recognised.
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